Saturday, October 10, 2015

Parallel Operation of Transformers; Conditions for Parallel operation of Transformers

Transformers are backbone of
electrical systems in any industry. There is always advisable to use
Transformers in parallel. There are following reasons why transformers are
advisable to use Transformers in Parallel:-

If numbers of Transformers are connected in parallel
then power supply from other transformers can be restored even if one no.
transformer get faulty. Also it becomes easy to do maintenance transformers by
taking out one by one without interruption of power supply, So overall
availability of electrical system increases.

2.To improve efficiency of Electrical
system:-

Transformer efficiency is always
maximum at full load. When numbers of transformers are connected in parallel
then load can be switched to only Transformers which will cater the total load
by running them nearer to full load. When load increases further other parallel
connected transformers can be connected in line. By this electrical system
efficiency can be improved.

3.To provide flexibility in electrical
system:-

If in future load demand increases or decrease it
becomes easy to add and remove Transformers instead of changing whole
electrical system transformer. This way it provides more flexibility to
electrical systems if transformers are connected in parallel.

If load increases in system then it becomes
economical to add a Transformer as per requirements in parallel with previous
transformer instead of replacing whole transformer, As bigger Transformer will
add to additional cost and also non-utilization of previous transformer.

As you have seen that there is
advisable to use Transformers in parallel. There are certains parameters which
must be kept in mind for parallel operation of Transformers.

Conditions for parallel operation
of Transformers are:-

1.Transformers should have same Voltage ratio

2.Transformers should have same impedance and X/R ratio.

3.Transformers which are to be connected in
Parallel should have same polarity.

4.Transformers undergoing parallel operation
should have same phase sequence.

1. Transformers
should have same voltage ratio:-

Transformers undergoing parallel operation should have same
voltage ratios. Now let’s take an example where transformers under parallel
operation have same primary voltages but have different secondary currents,
when they are connected in parallel there will be circulating currents in
secondary and which will reflects in primary also. As Transformer internal
impedance is very small this will leads to sufficiently high circulating
currents and leads to high I2R losses.

2. Transformers should have same Impedance and
X/R ratio:-

When Transformers
same impedance then they share load according to their ratings. But if there is
different X/R ratio then there will be difference in phase angle of current
carried by Transformers which will leads to one Transformer working at Higher
power factor then other. This will leads to unequal sharing of load of
Transformers.

Thus we can easily say from above that Impedance is inversely
proportion to Transformer rating. This means if transformer has same percentage
impedance or per unit impedance then parallel operation will be done.

In case of single phase Transformers in Three phase banks it
becomes very difficult of match Transformer impedances, so it is always good practice
to match X/R ratio also for parallel operation of Transformers.

3. Transformers which are to be connected in Parallel should have same
polarity.

Transformers which are
undergoing parallel operation should have same voltage polarity at its output.
If Transformers induced voltages have opposite polarity the this will leads to
very high circulating currents and may even leads to short circuit at its
output.

4. Transformers undergoing parallel operation
should have same phase sequence.

For parallel operation
Transformers should have same phase sequence. If this not done then this will
leads to short circuits. This condition is of utmost importance for parallel
operation of Transformers.

Thus
we have seen the conditions for parallel operation of Transformers.